Process of formation of large-ignimbrite plateaus

在日本九州南部广泛分布着凝灰岩高原。高原主要由大型伊东火山碎屑流沉积物组成，这是大约22，000年前在Aira破火山口大喷发的产物。伊东熔结凝灰岩区被选为研究大型熔结凝灰岩高原形成的代表性实例。伊东熔结凝灰岩高原的顶面分为两类：熔结凝灰岩的原始沉积面和具有源自伊藤熔结凝灰岩的水成（改造）沉积物的河流面。后者可分为可能为片流成因的沉积物的上表面和河流阶地表面，片流沉积物广泛分布于伊藤熔结凝灰岩高原的顶部，呈水平层状，厚度通常小于3 m。河流阶地沿着伊藤熔结凝灰岩区的主要河流分布。阶地沉积物的厚度通常小于几米，火山灰层总厚度为几米，均匀地覆盖着沉积物和河流表面。改造矿床正顶部最低火山灰的年龄约为18，000 y. B. P.，这表明改造矿床形成于18，000 y. B. P.~ 22，000 y. B. P.之间。一般认为，火山碎屑流侵位后，由于缺乏植被，原始沉积面遭受的切割最为剧烈。未焊接熔结凝灰岩的极高可蚀性和上述年龄表明，改造矿床和熔结凝灰岩高原的形成可能是在火山碎屑流就位后的很短时间内迅速完成的。

Ignimbrite plateaus are distributed extensively in southern Kyushu, Japan. The plateaus are composed mainly of large Ito pyroclastic flow deposits, which is a product of big eruptions at Aira caldera about 22,000 y.B.P. The Ito-ignimbrite field was selected as a representative example for study of formation of large-ignimbrite plateaus.The top surface of the Ito-ignimbrite plateaus are classified into two: the original depositional surface of the ignimbrite and the fluvial surfaces with water-laid(reworked) deposits derived from Ito ignimbrite. The latter is divided into the upper surface with deposits of probable sheet flood origin and the fluvial terrace surfaces.The sheet flood deposits are widely distributed on the top of the Ito-ignimbrite plateaus, horizontally stratified, and usually less than 3 meters in thickness. fluvial terraces are distributed along main rivers in the Ito-ignimbrite field. Thickness of the terrace deposits is usually less than several meters.Tephra layers several meters in total thickness uniformly cover both the depositional and the fluvial surfaces. The age of the lowermost tephra on the immediate top of the reworked deposits is about 18,000 y.B.P. This indicates that the reworked deposits were formed between 18,000 y.B.P. and 22,000 y.B.P.It is generally considered that original depositional surface immediately after the emplacement of pyroclastic flows suffers most violent dissection because of lack of vegetation. Very high erodibility of the unwelded ignimbrite and the above-mentioned ages suggest the probability that the formation of the reworked deposits and the ignimbrite plateaus were accomplished rapidly in a very short period immediately after the emplacement of pyroclastic flows.